asfdec: Check the return value of asf_read_stream_properties

This makes sure errors in setting stream parameters are passed
on to the caller. This avoids successfully opening files while
some parameters aren't filled in properly.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>